1. Field of the Invention
The present invention relates to a zoom lens system which is used in a small, light-weight video camera or a digital camera, etc., which has a half angle-of-view of 30° or more at the short focal length extremity, a zoom ratio of about 2, a simple structure, and which can be produced at low costs.
2. Description of the Prior Art
In recent years, along with further miniaturization of miniature imaging devices such as CCDs and higher density thereof, the zoom lens system used in a video camera and an electronic still camera or the like has also been required to reduce in size and to have an increased optical performance. Furthermore, in a solid-state image sensor used in such a video camera or electronic still camera, a color-separation filter is provided in the close vicinity of a light-receiving surface. Due to this arrangement, when a bundle of light rays passed through the photographing lens system is incident on the light-receiving surface at an inclined angle with respect to the light-receiving surface, the light is obscured (shaded) due to the color-separation filter. If shading occurs, such shading causes a decrease of peripheral illumination, and color irregularities occur due to deviation of the positional relationship between the color-separation filter and the pixels on the light-receiving surface. Accordingly, in order to attain ‘optimum telecentricity’, i.e., the bundle of light rays being incident on the light-receiving surface at an angle close to a right angle (90°) with respect to the light-receiving surface, an optical system in which the exit pupil is as far away from the image plane is required.
Furthermore, in recent years, in zoom a compact camera, although it is important for the camera to be smaller in a photographing position, compactness of the camera when the photographing lens system is retracted and is carried around has also become an important factor for further miniaturization. In other words, in such a zoom lens system, upon the photographing lens system is accommodated, i.e., when the photographing lens system is in the retracted position, a thin camera body is desirable. In order to achieve such a thin camera body, it is necessary to reduce the thickness of the lens groups constituting the zoom lens system, and to reduce the traveling distance of each lens group upon zooming for reducing mechanical burden.
As a miniature zoom lens system of the prior art, a two-lens-group zoom lens system constituted by a negative first lens group and a positive second lens group is known. However, in such a two-lens-group zoom lens system, the exit pupil position is relatively close to the image plane, which is not desirable for use with solid-state image sensor.
In order to solve the above-mentioned problem, as a zoom lens system having a smaller number lens elements and being produced at low costs, Japanese Unexamined Patent Publication (JUPP) Nos. 10-206732 and 11-211984 disclose the two-lens-group zoom lens system which has improved telecentricity by providing a stationary or movable positive lens group having a relatively strong refractive power between the second lens group and the imaging device.
In the two-lens-group zoom lens system taught in JUPP No. 10-206732, a zooming optical system is simply constituted by five lens elements. However, the zoom lens system disclosed therein has the following disadvantages:
(i) the aperture ratio at the short focal length extremity is large, i.e., 4 or more; and
(ii) zooming is performed discontinuously, i.e., the zoom lens system is not arranged to perform continuous zooming.
Furthermore, the three-lens-group zoom lens system disclosed in JUPP No. 11-211984 achieves a zoom ratio of about 2 with a simple lens arrangement of the six lens elements. However, the half angle-of-view at the short focal length extremity is narrow, i.e., about 25°, and aspherical lens elements requiring high machining technology are employed in the first and third lens groups. Accordingly, the zoom lens system disclosed therein is not satisfactory from the viewpoint of a wider angle-of-view, and reducing costs; and the zoom lens system therefore still has room for improvement.